1. Field of the Invention
The present invention concerns a semiconductor device comprising, for example, a group III nitride compound semiconductor and a method of manufacturing thereof, as well as a light emitting semiconductor device
2. Description of the Related Art
In recent years, vigorous development has been conducted for light emitting semiconductor devices such as semiconductor lasers or light emitting diodes (LED) capable of emitting light in a range from a visible region to a UV-region by using Group III nitride compound semiconductors such as AlGaInN. Particularly, in the field of light recording, it has been demanded for attain practical use of a semiconductor laser capable of obtaining a light in a short wavelength region in order to improve the recording density, for example, of an optical disc.
Recently, in AlGaInN system semiconductor lasers, continuous oscillation for 300 hours at a room temperature has been attained by growing a layer comprising a Group III nitride compound semiconductor by way of a buffer layer on a substrate made of sapphire by means of a metal organic chemical vapor deposition (MOCVD) method (Jpn. J. Appl. Phys. 35, L74 (1996); and 36 L1059 (1997)). However, in view of the progress curve of a driving current during use, moderate increase is observed from the initial stage of voltage supply and it can be seen that degradation proceeds gradually. As the cause for the degradation, it is considered that the layer comprising the Group III nitride compound semiconductor formed on a substrate contains threading dislocations (dislocations in which dislocation defects are propagated and penetrate crystals) of about 1.times.10.sup.8 -1.times.10.sup.9 N/cm.sup.2. Accordingly, it is necessary to reduce the density of threading dislocations in order to attain a practical working life of 10,000 hours or more, for which various studies have been made.
For example, one of such counter measures include, for example, a method of forming a GaN layer by way of a buffer layer on a sapphire substrate, laminating thereon a mask layer in which mask areas comprising silicon dioxide (SiO.sub.2) strips each of 1 to 4 .mu.m with at a pitch of 7 .mu.m and then selectively growing a GaN layer laterally by a halide vapor deposition layer on the mask layer (Jpn. J. Appl. Phys, 36 L899 (1997)). According to this method, the density of the threading dislocations in the GaN layer formed on the mask layer can be reduced to about 1.times.10.sup.7 N/cm.sup.2,
In this method, however, unevenness is liable to be caused on the surface of the GaN layer formed on the mask layer and it is difficult to obtain a planar surface. This is because, the GaN layer at first proceeds in the opening areas of the mask layer (that is, between each of the mask areas) to form a protuberance and then proceeds to a portion above the mask areas (that is, in the lateral direction) in the course of growing. Accordingly, in order to make the surface planar, it is necessary to increase the thickness of the GaN layer to at least 10 .mu.m or more. Accordingly, this requires a long time for growing and results in various problems that defects or warps are formed due to disagreement of the lattice constant relative to the substrate made of sapphire.
Further, although propagation of the threading dislocation can be suppressed by selective growing in the lateral direction above the mask area, the threading dislocation from the GaN layer below the mask layer is continued as it is in the opening areas. Therefore, while the density of the threading dislocations is reduced in the regions above the mask areas, the density of the threading dislocations is maintained above the opening areas and, thus, the density can not be reduced as a whole. Accordingly, a light emitting region has to be formed exactly in the regions above the mask areas, which results in a problem that the degree of freedom in the manufacture is small, the manufacturing steps are complicated and manufacture is made difficult.
For the technique of selectively growing the GaN layer on the mask layer, a MOCVD method has also been reported in addition to the halide vapor deposition method (J. Crtst. Growth 144 133 1994)). However, this is not a report aiming at the selective growing on the mask layer. Further, a report examining the anisotropy upon selective growing on the mask layer by the MOCVD method has been made recently. According to this report, when a GaN layer is formed by way of a buffer layer on a substrate made of sapphire and a GaN layer is formed thereover by way of a mask layer in which a plurality of strip-like mask areas are formed in &lt;11-20&gt; direction, lateral growing is accelerated and growing of protuberance in the opening areas is suppressed to obtain a relatively planar grown surface closer to a C face under certain conditions (Appln. Phys. Lett 71 1204 (1997)). However, the report does not mention to the dislocation density but only shows the possibility of satisfactory crystal growing in the lateral direction.
The &lt;11-20&gt; direction mentioned herein should, exactly, be expressed by attaching an overline above a numerical figure as shown below but this is indicated herein by attaching "-" before the numerical figure (hereinafter, this convenient indication is used for such expression). EQU &lt;1120&gt;